1. Field of the Invention
The present invention relates to a DS-CDMA (Direct Sequence-Code Division Multiple Access) system which is one of CDMA communication systems, and more particularly to a multiuser interference canceler, which is used in a DS-CDMA system, for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user.
2. Description of the Related Art
In recent years, CDMA communication systems that are resistant to interference and jamming have attracted much attention as mobile communication systems. In a CDMA communication system, a transmitting station spreads a user signal to be transmitted with a spreading code and transmits the spread user signal, and a receiving station despreads the received user signal with a spreading code which is the same as the spreading code in the transmitting station for thereby obtaining the original user signal.
In the CDMA communication system, a plurality of transmitting stations spread user signals with different orthogonal spreading codes, and a plurality of receiving stations select respective spreading codes for use in despreading the received user signals for thereby specifying the respective user signals. Consequently, a common frequency band can be used to transmit a plurality of the user signals from the transmitting stations to the receiving stations.
However, because not all spreading codes are made orthogonal to each other, the signal of a user may possibly serve as noise with respect to another user, resulting in interference. Therefore, if a plurality of users use a link with a common frequency within one cell, the signals of the users tend to interfere with each other, resulting in a reduction in the bit energy to noise ratio (Eb/No) which is the ratio of the desired reception wave power to interfering wave power. Because the Eb/No determines the quality of communications, it is necessary to attain an Eb/No having at least a certain value in order to obtain a desired quality of communications. Such an Eb/No having at least a certain value is referred to as a required Eb/No.
The number of links that can use a common frequency within one cell is limited to a number that can maintain the required Eb/No. For increasing a system capacity for subscribers, any interference between communication links must be lowered.
One proposed method of reducing any interference within cells, increasing a system capacity for subscribers, or improving the quality of communications in the DS-CDMA communication system is a multiuser interference canceling method which employs a multiuser interference canceler.
The multiuser interference canceling method is a method in which when a received signal containing the signals of a plurality of users is to be demodulated by a spreading code of a certain user, an interference replica component which is the same as a signal component other than the user is generated and subtracted from the received signal to be demodulated in a multiplicity of stages for thereby reducing interference from the other users. There are two types of multiuser interference cancelers, i.e., a serial multiuser interference canceler and a parallel multiuser interference canceler. The principles of the serial multiuser interference canceler are described in Technical Report (RC95-50) of the Institute of Electronics, Information and Communication Engineers, xe2x80x9cSequential channel estimating serial canceler using a pilot symbol in DS-CDMAxe2x80x9d, for example. Japanese laid-open patent publication No. 09-270736 (JP, 09270736, A) entitled xe2x80x9cDC-CDMA multiuser serial interference canceler devicexe2x80x9d discloses a type of serial multiuser interference canceler.
The conventional serial multiuser interference canceler disclosed in the above publication is shown in FIG. 1.
The serial multiuser interference canceler shown in FIG. 1 is designed for three users and comprises first, second, and third stages for effecting an interference cancellation process with an interference replica signal and three decoders 19a-19c. Each of the first, second, and third stages comprises three blocks. Therefore, the serial multiuser interference canceler has a total of nine blocks.
These nine blocks have respective interference canceler units (ICUs) 61a, 61b, 61c, 62a, 62b, 62c, 63a, 63b, 63c. Each of the blocks also has, in addition to the ICU, a delay memory (D) and a subtractor and/or an adder. For example, the first block of the first stage comprises an ICU 61a, a delay memory 3, and a subtractor 4, and the first block of the second stage comprises an ICU 62a, delay memories 5 and 7, an adder 8, and a subtractor 6. Each of the second and third blocks of the first stage is identical in structure to the first block of the first stage. Each of the blocks of the second and third stages, except the third block of the third stage, is identical in structure to the first block of the second stage.
Each of the ICUs 61a, 62a, 63a generates an interference replica signal which is the same as the signal component of the first user. Each of the ICUs 61b, 62b, 63b generates an interference replica signal which is the same as the signal component of the second user. Each of the ICUs 61c, 62c, 63c generates an interference replica signal which is the same as the signal component of the third user.
The arrangement of the ICU 61a, as an example of the ICUs 61a-63c, will be described below with reference to FIG. 2.
The ICU 61a comprises a plurality of despreaders xe2x88x92711-71n, a rake combiner 26, a decision unit 27, and a plurality of respreaders 721-72n. Each of the despreaders 711-71n comprises a pair of multipliers 22 and 25, an integrator 23, and a transmission path estimator 24. A received signal 1 that is supplied to the ICU 61a is multiplied by a spreading code Ca by the multiplier 22, and then integrated by the integrator 23, which produces a correlated value. The transmission path estimator 24 determines a transmission path fading vector "xgr" from the correlated value determined by the integrator 23. The multiplier 45 multiplies the correlated value from the integrator 23 by the inverse "xgr" of the transmission path fading vector "xgr" from the transmission path estimator 24 for thereby performing phase correction for the received signal.
The received signals from paths which have been corrected in phase by the multipliers 25 of the despreaders 711-71n are combined by the rake combiner 26, and the combined signal is decoded into an original symbol sequence by the decision unit 27. Since the rake combiner 26 and the decision unit 27 are of general nature in the CDMA communication system and do not have direct bearing on the operation of the multiuser interference canceler, they will not be described in detail below. However, those skilled in the art will be able to construct the rake combiner 26 and the decision unit 27 with ease.
Each of the respreaders 721-72n comprises a pair of multipliers 28 and 29. In each of the respreaders 721-72n, the multiplier 28 multiplies the original symbol sequence by the transmission path fading vector "xgr" of one of the paths to impart original transmission path characteristics to the original symbol sequence. Thereafter, the original symbol sequence is spread with the spreading code Ca by the multiplier 49. The signals from the respreaders 721-72n are combined into a chip-rate interference replica signal 81a, which is outputted from the ICU 61a. 
In FIG. 2, the signal supplied from the rake combiner 26 to the decision unit 27 is branched and outputted to an external circuit. Such a signal branching arrangement is included in only the ICUs 63a, 63b, 63c in the third stage. From the ICUs 63a, 63b, 63c, the signals are supplied to the decoders 19a, 19b, 19c, respectively.
For the sake of brevity, operation of the conventional serial multiuser interference canceler shown in FIG. 1 for demodulating the signal of the first user will be described below. The signals of the other users will be demodulated in the same manner as described below.
The ICU 61a is supplied with the received signal 1, generates a signal which is the same as the signal component of the first user contained in the received signal 1, and outputs the generated signal as an interference replica signal 81a. The delay memory 3 stores the received signal 1 and then outputs the received signal 1 after having delayed it for a certain time. The time for which the delay memory 3 delays the received signal 1 is a time required for the ICU 61a to generate the interference replica signal 81a. The subtractor 4 subtracts the interference replica signal 81a from the received signal 1 outputted from the delay memory 3, and then outputs the differential signal. Therefore, the differential signal outputted by the subtractor 4 contains only the signal of the second user and the signal of the third user.
In the second block of the first stage, the subtractor 4 subtracts the signal of the second user from the signal outputted from the delay memory 3. In the third block of the first stage, the subtractor 4 subtracts the signal of the third user from the signal outputted from the delay memory 3.
Upon completion of the processing in the first stage, therefore, the interference replica signals of all the users have been subtracted from the received signal 1, leaving a residual signal, which is supplied to the first block of the second stage.
In the first block of the second stage, the adder 8 adds the interference replica signal 81a which has been delayed by the delay memory 7 for a certain time, to the residual signal from the first stage. Therefore, the output signal from the adder 8 contains only the signal component of the first user. The ICU 62a is supplied with the output signal from the adder 8, and generates and outputs an interference replica signal 82a. The output signal from the adder 8 is also supplied to the delay memory 5, which delays the signal for a certain time and outputs the delayed signal to the subtractor 6. The subtractor 6 subtracts the interference replica signal 82a from the delayed signal from the delay memory 5.
The signal outputted from the subtractor 6 after the processing in the first block of the second state is a residual signal that does not contain any signals of the users.
The same processing as described above is carried out in the second block of the second stage through the third block of the third stage.
In each of the blocks of the third stage, the ICUs 63a-63c output signals from the rake combiners 26 thereof to the respective decoders 19a-19c. The decoders 19a through 19c finally decode the supplied signals.
The signals handled in the first to third stages of the conventional serial multiuser interference canceler are all chip-rate signals.
The conventional parallel multiuser interference canceler will be described below with reference to FIG. 3. The parallel multiuser interference canceler shown in FIG. 3 is also designed for three users as with the serial multiuser interference canceler shown in FIG. 1. Those parts of the parallel multiuser interference canceler shown in FIG. 3 that are identical to those shown in FIG. 1 are denoted by identical reference numerals.
As shown in FIG. 3, the parallel multiuser interference canceler comprises first, second, and third stages, and three decoders 19a-19c. The first stage comprises a delay memory 51, three ICUs 61a-61c, an adder 57, a subtractor 54, and three adders 58a-58c. The second stage comprises a delay memory 52, three ICUs 62a-62c, an adder 59, a subtractor 55, and three adders 60a-60c. The third stage comprises a delay memory 53, three ICUs 63a-63c, an adder 64, a subtractor 56, and three adders 65a-65c. 
In the first stages, the ICUs 61a-61c, which are connected parallel to each other, generate respective interference replica signals 81a-81c which are the same as the signal components of the first to third users. The interference replica signals 81a-81c are combined by the adder 57 into a signal that is subtracted by the subtractor 54 from a received signal 1 that has been delayed for a certain time by the delay memory 51. Therefore, the subtractor 54 outputs a residual signal that is free of the interference replica signals of all the users. The residual signal is then added to the interference replica signals 81a-81c in the first stage by the adders 58a-58c. The interference replica signals 81a-81c to which the residual signal has been added are supplied to the respective ICUs 62a-62c of the second stage.
The second and third stages also perform the same processing operation as the processing operation, described above, of the first stage. The decoders 19a-19c finally decode respective signals supplied from the third stage.
In the above conventional multiuser interference canceler, the interference replica signal is estimated and reproduced from the received signal, and the interference replica signal is subtracted from the received signal. Therefore, the accuracy with which the interference replica signal is reproduced significantly affects the characteristics of the interference canceler. Factors which are responsible for the accuracy with which the interference replica signal is reproduced are the accuracy of the transmission path fading vector "xgr" extracted by the transmission path estimator 24 in each ICU and the error ratio of the hard decision result carried out by the decision unit 27 after the rake combining.
The transmission path fading vector "xgr" represents an estimation of characteristics added on the transmission path of the interfering user, and the hard decision value represents an estimation of the transmitted sequence. Either one of the transmission path fading vector "xgr" and the hard decision value is closely related to the ratio of desired signal power to noise power of the received signal. As the ratio of desired signal power to noise power is deteriorated, the error of the transmission path fading vector "xgr" and the error rate of the hard decision value are rendered poorer. Therefore, the accuracy with which the interference replica signal is reproduced is also reduced. While the ratio of desired signal power to noise power may be expressed as xe2x80x9cSN ratioxe2x80x9d, xe2x80x9cSIRxe2x80x9d, xe2x80x9cEb/Noxe2x80x9d, etc., it will be expressed as xe2x80x9cEb/Noxe2x80x9d herein.
The conventional multiuser interference cancelers have a problem in that since interference replica signals are uniformly reproduced for interference cancellation even when the Eb/No is low, the reception quality is degraded in regions where the Eb/No of the received signal is low.
In the actual environments of mobile communication systems, furthermore, the reception quality is sometimes degraded by time-dependent changes due to fading of individual signals and external noise when the multiuser interference canceler is operated in regions where the Eb/No of the received signal is not significantly low.
Consequently, the conventional multiuser interference cancelers have suffered the following disadvantages:
(1) The reception quality is degraded when interference cancellation is performed in regions where the Eb/No of the received signal is low.
(2) If the received signal suffers time-dependent changes due to fading of individual signals and external noise, then the reception quality may be also degraded when the multiuser interference canceler is operated.
It is an object of the present invention to provide a multiuser interference canceler which is capable of preventing reception quality from being degraded in all operating environments.
According to a first aspect of the present invention, there is provided a DS-CDMA multiuser interference canceler for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user, comprising a first stage comprising a plurality of series-connected blocks having respective first interference canceler units associated respectively with the users, for generating and outputting interference replica signals which are identical to signal components of the users, and respective first subtracting means for subtracting the interference replica signals from the received signal, a plurality of second stages comprising a plurality of series-connected blocks having respective adding means associated respectively with the users, for adding interference replica signals of the users generated in a preceding stage to signals from a preceding block, respective second interference canceler units for generating again interference replica signals which are identical to the signal components of the users from output signals from the adding means, and respective second subtracting means for subtracting the interference replica signals which have been generated again from the output signals from the adding means and outputting differential signals to a next block, a plurality of decoding means for decoding symbols generated in a final one of the second stages, and a plurality of preliminary demodulating means for measuring reception quality of the signals of the users contained in the received signal, each of the first and second interference canceler units comprising means for controlling an interference canceler unit in a preceding block to turn off the outputting of the interference replica signal if the reception quality of output signals from the preceding block is more degraded than the reception quality of the signals of the users which has been measured by the preliminary demodulating means.
In the above DS-CDMA multiuser interference canceler, which is of the serial type, if the reception quality of output signals from the preceding block is more degraded than the reception quality of the signals of the users which has been measured by the preliminary demodulating means, then each of the interference canceler units determines that the reception quality has been degraded by an interference cancellation process in the preceding block, and controls the interference canceler unit in the preceding block to turn off the outputting of the interference replica signal. The DS-CDMA multiuser interference canceler is therefore capable of preventing the reception quality from being degraded by an interference cancellation process.
According to a second aspect of the present invention, there is provided a DS-CDMA multiuser interference canceler for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user, comprising a first stage comprising a plurality of series-connected blocks having respective first interference canceler units associated respectively with the users, for generating and outputting interference replica signals which are identical to signal components of the users, and measuring reception quality of the signals of the users, and respective first subtracting means for subtracting the interference replica signals from the received signal, a plurality of second stages comprising a plurality of series-connected blocks having respective adding means associated respectively with the users, for adding interference replica signals of the users generated in a preceding stage to signals from a preceding block, respective second interference canceler units for generating again interference replica signals which are identical to the signal components of the users from output signals from the adding means and measuring reception quality of the signals of the users, and respective second subtracting means for subtracting the interference replica signals which have been generated again from the output signals from the adding means and outputting differential signals to a next block, a plurality of decoding means for decoding symbols generated in a final one of the second stages, a plurality of preliminary demodulating means for measuring reception quality of the signals of the users contained in the received signal, and cancellation on/off central control means for controlling an interference canceler unit in an either one of the stages preceding the one of the second stages to turn off the outputting of the interference replica signal if the reception quality measured by the second interference canceler units in one of the second stages is more degraded than the reception quality of the users measured by the preliminary demodulating means, until the reception quality measured by all the interference canceler units in said stage becomes better than the reception quality of the users measured by the preliminary demodulating means.
In the above DS-CDMA multiuser interference canceler, which is of the serial type, all the information of the reception quality measured by the interference canceler units is supplied to the cancellation on/off central control means, which makes an appropriate decision to determine an interference canceler unit which is to turn off an interference cancellation process. Therefore, the turning on and off of the interference cancellation process in the interference canceler units in each block can adequately be controlled. The interference canceler unit controlled to turn off the outputting of the interference replica signal should preferably be one of the interference canceler units, whose reception quality is worst, in a stage preceding said stage.
According to a third aspect of the present invention, there is provided a DS-CDMA multiuser interference canceler for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user, comprising a first stage comprising a plurality of first interference canceler units associated respectively with the users, for generating and outputting interference replica signals which are identical to signal components of the users, and measuring reception quality of the signals of the users, first adding means for adding the interference replica signals, and first subtracting means for subtracting an output signal of the first adding means from the received signal, a plurality of second stages comprising a plurality of second adding means associated respectively with the users, for adding interference replica signals of the users generated in a preceding stage to signals from a preceding stage, respective second interference canceler units for generating again interference replica signals which are identical to the signal components of the users from output signals from the second adding means and measuring reception quality of the signals of the users, respective third adding means for adding the interference replica signals generated again by the second interference canceler units, and respective second subtracting means for subtracting output signals of the third adding means from the received signal and outputting differential signals to a next stage, a plurality of decoding means for decoding symbols generated in a final one of the second stages, a plurality of preliminary demodulating means for measuring reception quality of the signals of the users contained in the received signal, and cancellation on/off central control means for controlling an interference canceler unit in an either one of the stages preceding the one of the second stages to turn off the outputting of the interference replica signal if the reception quality measured by the second interference canceler units in one of the second stages is more degraded than the reception quality of the users measured by the preliminary demodulating means, until the reception quality measured by all the interference canceler units in said stage becomes better than the reception quality of the users measured by the preliminary demodulating means.
In the above DS-CDMA multiuser interference canceler, which is of the parallel type, all the information of the reception quality measured by the interference canceler units is supplied to the cancellation on/off central control means, which makes an appropriate decision to determine an interference canceler unit which is to turn off an interference cancellation process. Therefore, the turning on and off of the interference cancellation process in the interference canceler units in each block can adequately be controlled.
Rather than simply turning off the interference cancellation process which is liable to degrade the reception quality, the output level of the interference replica signal may be reduced for making the reception quality more uniform.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.